Object information acquiring apparatus and object information acquiring method

ABSTRACT

This invention employs an object information acquiring apparatus including a probe for receiving, as a received signal, an acoustic wave which is generated within an object irradiated with light and propagates on an object surface, and a processor for generating object information, which is information based on an internal optical characteristic value of the object, by using intensity of the received signal. The processor corrects the intensity of the received signal by using the reflectance upon the acoustic wave entering the probe which is calculated based on the angle of the acoustic wave entering the probe, and on the acoustic impedance and acoustic velocity of the object and the probe.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an object information acquiring apparatus andan object information acquiring method.

2. Description of the Related Art

The research of optical imaging technology capable of obtaininginformation in a living subject by irradiating light to the livingsubject from a light source such as a laser is being actively pursued inthe medical field. Photoacoustic Tomography (PAT) is one of the opticalimaging technologies. A photoacoustic tomography device irradiatespulsed light generated from the light source to the living subject, anduses a probe to receive, in various positions, an acoustic wave(typically an ultrasound wave) that is generated when the body tissue,which absorbed the light energy that propagated and diffused in theliving subject, instantaneously expands. It is thereby possible todetect, for instance, the difference in the absorption factor of lightenergy in the suspected site of a tumor or the like and other tissues.

Subsequently, the image reconfiguration region (region where informationin the living subject is to be imaged) is divided into a plurality ofvoxels or pixels. In addition, image reconfiguration is performed byusing the received signal of the timeframe in which the acoustic wavereaches the elements of the probe from the voxels or pixels. For theimage reconfiguration process, FBP (Filtered Backprojection),DELAY-AND-SUM and other well-known methods may be used. It is therebypossible to obtain the optical characteristic value distribution such asthe initial sound pressure distribution or the absorption coefficientdistribution in the living subject. As a result of performingphotoacoustic measurement using light of various wavelengths, it ispossible to quantitatively observe a specific substance in the object;for example, measure the hemoglobin concentration contained in the bloodor measure the oxygen saturation of the blood.

In Photoacoustic Tomography, a probe is used to receive the acousticwave. In the field of ultrasound diagnostic devices that similarlyreceive acoustic waves using a probe, it is known that the receivingsensitivity of the probe depends on the incidence angle of the acousticwave that enters the probe (NPL 1: “Ultrasound Wave Diagnostic Device”Co-authored by Masayasu Ito and Tsuyoshi Mochizuki, Published by CoronaPublishing Co., Ltd., Aug. 26, 2002). In other words, as the incidenceangle of the acoustic wave that enters the probe increases, thereceiving sensitivity of the probe decreases, and there is a problem inthat imaging of favorable directionality cannot be performed.

Thus, Japanese Patent Application Publication No. S60-021744 (PTL 1)discloses a method of correcting the differences in the receivingsensitivity caused by the incidence angle. In PTL 1, the probe transmitsthe acoustic wave at a certain angle, and receives the reflected wavefrom the object. In addition, the sensitivity difference which arisesaccording to the incidence angle of the acoustic wave is corrected bychanging the amplification factor for each element of the probe, and thesensitivity of the respective elements is caused to be uniform so as toenable imaging of favorable directionality.

Patent Literature 1 (PTL 1): Japanese Patent Application Publication No.S60-021744

Non Patent Literature 1 (NPL 1): “Ultrasound Wave Diagnostic Device”Co-authored by Masayasu Ito and Tsuyoshi Mochizuki, Published by CoronaPublishing Co., Ltd., Aug. 26, 2002

SUMMARY OF THE INVENTION

In the field of Photoacoustic Tomography also, there is a problem inthat the receiving sensitivity of the probe differs depending on theincidence angle of the acoustic wave. Even when acoustic waves havingthe same intensity are used, the acoustic wave is detected to be smallerwhen the acoustic wave enters the probe diagonally in comparison to acase where the acoustic wave enters the probe perpendicularly. Thus, thevisual angle (aperture angle) from the arbitrary voxel or pixel to theelement apparently decreases. When the aperture angle decreases, theresolution of the reconstructed image will deteriorate. Based on theprinciple of Photoacoustic Tomography, while it is ideal to reconfigurethe image by receiving acoustic waves at all circumferences of the lightabsorber so as to accurately learn the size of the light absorber, thereconstructed image will become blurry when the range of receiving theacoustic waves is limited.

To deal with this problem, the field of ultrasound wave diagnosisemploys the method of PTL 1. Nevertheless, with PhotoacousticTomography, since it is not possible to know from which angle theacoustic wave was propagated when acoustic wave is received, it is notpossible to use the method of PTL 1. Even if the method of PTL 1 isapplied to Photoacoustic Tomography, it would be inappropriate sincesignals other than those of a timeframe in which signals should bereceived from the voxels or pixels to be subject to imagereconfiguration are also amplified.

Moreover, the sensitivity of the probe is determined based on theaperture of the probe and the reflectance loss on the probe surface.Here, the relationship of the aperture of the probe and the reflectanceloss on the probe surface is explained.

Foremost, FIG. 12 shows a table of the acoustic impedance of the bodytissues (Source: NPL 1, p. 13, Table 2.1). As evident from this table,the acoustic impedance of body tissues differs based on the propagatingmedium.

FIG. 10 is a combination of a graph (solid line) showing therelationship of the incidence angle of the acoustic wave entering theprobe and the transmittance of the acoustic wave into the probe, and agraph (respective dotted lines) showing the relationship of theincidence angle of the acoustic wave entering the probe and thereceiving sensitivity of the probe for each element size of the probe.Here, the acoustic wave frequency was set to 1 MHz. The additionalsettings were as follows; namely, the acoustic impedance Z₁ of theobject=1.5×10⁶ kg/m²s, the acoustic velocity c₁ of the object=1500 m/s,the acoustic impedance Z₂ of the probe (acoustic matching layer of theprobe surface)=1.8×10⁶ kg/m²s, and the acoustic velocity c₂ of theprobe=2100 m/s.

Upon viewing the graph, when the element size of the probe is 1 mm ormore, the deterioration in sensitivity caused by the aperture of theprobe is dominant in comparison to the deterioration in sensitivitycaused by the reflectance loss on the probe surface. However, when theelement size of the probe becomes 0.5 mm or less, the deterioration insensitivity caused by the reflectance loss on the probe surface becomesdominant. Accordingly, particularly when the element size of the probeis small, the deterioration in sensitivity of the probe caused a problemby the reflectance loss on the probe surface.

This invention was devised in view of the foregoing problems. Thus, anobject of this invention is to improve the resolution of thereconstructed image by correcting the reflectance loss at the interfaceof the object and the probe in Photoacoustic Tomography.

This invention provides an object information acquiring apparatus,comprising:

a probe for receiving, as a received signal, an acoustic wave which isgenerated within an object irradiated with light and propagates on anobject surface; and

a processor for generating object information, which is informationbased on an internal optical characteristic value of the object, byusing intensity of the received signal,

wherein the processor corrects the intensity of the received signal byperforming amplification corresponding to a reflectance loss of theacoustic wave which is obtained according to an angle of the acousticwave entering the probe.

This invention also provides an object information acquiring method,comprising:

a receiving step of a probe receiving, as a received signal, an acousticwave which is generated within an object irradiated with light andpropagates on an object surface; and

a processing step of a processor generating object information, which isinformation based on an internal optical characteristic value of theobject, by using intensity of the received signal,

wherein, in the processing step, the intensity of the received signal iscorrected by performing amplification corresponding to a reflectanceloss of the acoustic wave which is obtained according to an angle of theacoustic wave entering the probe.

According to this invention, it is possible to improve the resolution ofthe reconstructed image by correcting the reflectance loss at theinterface of the object and the probe in Photoacoustic Tomography.

Further features of this invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram explaining the configuration of the photoacousticsignal acquiring apparatus of Embodiment 1;

FIGS. 2A and 2B are diagrams explaining the positional relationship ofthe light absorber and the respective elements of the probe;

FIGS. 3A and 3B are diagrams explaining the reflection of the acousticwave at the interface of the object and the probe;

FIG. 4 is a flowchart of the correction processing of Embodiment 1;

FIG. 5A is a diagram explaining the configuration of the photoacousticsignal acquiring apparatus of Embodiment 2;

FIG. 5B is a flowchart of the correction processing of Embodiment 2;

FIG. 6 is a flowchart of the correction processing of Embodiment 3;

FIG. 7 is a diagram explaining the configuration of the photoacousticsignal acquiring apparatus of Embodiment 4;

FIG. 8 is a diagram explaining the propagation of the acoustic wave inthe holding plate;

FIG. 9A is a diagram explaining the configuration of the photoacousticsignal acquiring apparatus of Embodiment 6;

FIG. 9B is a diagram explaining the method of measuring the acousticimpedance of the object of Embodiment 6;

FIG. 10 is a diagram explaining the incidence angle of the acoustic waveupon entering the probe;

FIGS. 11A and 11B are diagrams schematically showing the intensitydistribution in the cross section of the light absorber; and

FIG. 12 is a table showing the acoustic impedance of body tissues.

DESCRIPTION OF THE EMBODIMENTS

A photoacoustic signal acquiring apparatus is now explained taking theobject information acquiring apparatus of this invention as an example.Nevertheless, the target of this invention is not limited to the ensuingconfiguration. This invention can also be perceived as an objectinformation acquiring method for realizing the following functions, oras an object information acquiring program for realizing the followingfunctions by being supplied to an information processing device(computer or the like) via a network or a storage medium.

(Photoacoustic Signal Acquiring Apparatus)

The photoacoustic signal acquiring apparatus of this invention is anapparatus for generating information (object information) in the objectbased on calculation. The photoacoustic signal acquiring apparatusincludes, as its basic hardware configuration, a light source, a probeas a receiver of acoustic waves, and a processor. Pulsed light emittedfrom the light source is irradiated to an object such as a livingsubject. When a part of the energy of light that propagated within theobject is absorbed by a light absorber (sound source) such as blood, anacoustic wave (typically an ultrasound wave, and also referred to asphotoacoustic wave or a photoacoustic ultrasound wave) is generatedbased on the thermal expansion of the light absorber. The acoustic waveis received by the probe and becomes an electric signal, and istransferred to the processor. The transferred electric signal isconverted into optical characteristic value distribution information orthe like in the object by the processor, and becomes object information.There is no particular limitation in the format of opticalcharacteristic value distribution information, and may be arbitrarilydetermined to be two-dimensional, three-dimensional or the likedepending on the purpose of measurement, device configuration, and soon. The generated object information may contain, in addition theoptical characteristic value distribution and the absorption coefficientdistribution, initial sound pressure distribution, substanceconcentration and oxygen saturation based thereon. In addition, it isalso possible to include image data for forming and displaying imagesbased on the foregoing information.

(Light Source)

In cases where the object is a living subject, irradiated from the lightsource is light of a specific wavelength that will be absorbed by aspecific component among the components configuring the living subject.The light source may be provided integrally with the photoacousticsignal acquiring apparatus, or the light source may be separated andprovided as a separate body. As the pulse width of the light source, apulse width of roughly 10 nanoseconds is used in order to efficientlygenerate photoacoustic waves. While a laser is preferably used as thelight source since significant power can be obtained, it is alsopossible to use a light-emitting diode, a flash lamp or the like insubstitute for a laser. As the laser, solid-state laser, gas laser, dyelaser, semiconductor laser and other lasers may be used. The timing,waveform and intensity of irradiation are controlled by a light sourcecontrol unit not shown. Note that the light source control unit may alsobe integrated with the light source. In this invention, the wavelengthof the used light source used is desirably a wavelength in which thelight will propagate to the inside of the object. Specifically, incaseswhere the inside of the object is a living subject, a wavelength of 500nm or more and 1200 nm or less is used.

(Probe)

A probe is a receiver for receiving acoustic waves that are generatedand propagate on the object surface and in the object to which thepulsed light was irradiated. The probe converts the received acousticwave into an electric signal (received signal), which is an analogsignal. Any probe may be used including a probe which uses thepiezoelectric phenomenon, a probe which uses the oscillation of light, aprobe which uses the change in capacitance, or any other probe so aslong as it can receive acoustic waves. If a component in which aplurality of receiving elements are disposed one-dimensionally ortwo-dimensionally is used as the probe, acoustic waves can be receivedsimultaneously in various positions, and it is thereby possible toshorten the measuring time. When there is only one receiving element, itis also possible to perform scanning using the probe and receive theacoustic waves in various positions. Moreover, since a probe normallycomprises an acoustic matching layer, the acoustic impedance, theacoustic velocity and the like of the probe to be used in the ensuingcorrection shall be the value of the acoustic matching layer on theoutermost surface of the probe.

(Processor)

The processor is configured from an information processing device suchas a computer and a circuit, and performs processing and operation ofelectric signals. The processor includes a conversion unit forconverting the electric signal obtained with the probe from an analogsignal into a digital signal. Desirably, the conversion unit can processa plurality of signals simultaneously. It is thereby possible to shortenthe time required for forming an image. The converted digital signal isstored in a memory.

The processor additionally includes a correction operation unit forcorrecting the reflectance loss at the interface between the object andthe probe based on the signals stored in the memory by using data andthe like stored in a table. In addition, the processor generates objectinformation such as optical characteristic value distribution, forinstance, via back projection with a time domain based on the correctedsignal.

The foregoing correction operation unit performs unique signalcorrection process in the image reconfiguration of arbitrary voxels orpixels; that is, processing of amplifying the intensity of the acousticwave signal. The processor amplifies the signals with a greatreflectance loss that enter the probe diagonally from the voxels or thepixels to a greater degree than the signals with a small reflectanceloss that enter the probe perpendicularly from the voxels or the pixels.Consequently, the angle distribution of the sensitivity differencecaused by the reflectance loss of acoustic waves on the probe surface isreduced, and deterioration in the resolution can be improved.

FIG. 11 is a diagram schematically showing how the intensitydistribution of the light absorber in the object changes depending onthe reflectance loss correction of this invention. FIG. 11A and FIG. 11Bare diagrams schematically showing the intensity distribution in thecross section of the light absorber before and after the reflectanceloss correction, respectively. In the diagrams, the horizontal axisrepresents the distance on the object, and the vertical axis representsthe signal intensity. Accordingly, it can be seen that the resolution isimproved by correcting the reflectance loss upon imaging the informationin the object.

The respective embodiments of this invention are now explained withreference to the drawings.

<Embodiment 1>

FIG. 1 is a diagram explaining the configuration of the photoacousticsignal acquiring apparatus of Embodiment 1 of this invention.

A light source 101 is a titanium-sapphire laser excited with a doublewave of a YAG laser, and generates a pulsed light 102. An irradiationoptical system 103 is configured, for example, from a lens, a mirror, anoptical fibre, and so on. The pulsed light 102 emitted from the lightsource 101 is guided by the irradiation optical system 103, andirradiated to the object 104. When a part of the energy of light thatpropagated inside the object 104 is absorbed by a light absorber 105such as blood, an acoustic wave 106 is generated and propagates in theobject 104. Apart of the acoustic wave that propagated in the directionof the probe 108 becomes a reflected wave 107, and the remainder isreceived by the probe.

The probe 108 receives the acoustic wave 106 with the elements andconverts the acoustic wave 106 into an analog electric signal, and sendsthe analog electric signal as a received signal 109 to the processor110. The processor 110 is configured from a conversion unit 110 a, amemory 110 b, a first table 110 c and a correction operation unit 110 d.The conversion unit 110 a amplifies the received signal 109 and performsA/D conversion to the received signal 109. The converted digital signalis stored in the memory 110 b. The first table 110 c stores at least twoamong acoustic impedance, density, and acoustic velocity of the objectand the probe. However, the foregoing data may be changed by an operatorvia an input means. The correction operation unit 110 d corrects thereflectance loss of the signals stored in the memory 110 b based on thedata input to the first table, and thereby performs imagereconfiguration.

In order to explain the reflectance loss correction processing as theunique feature to be performed by the correction operation unit 110 d,the received signal 109 is foremost explained with reference to FIG. 2.FIG. 2A is a diagram for explaining the positional relationship of thelight absorber 105 and the respective elements 108 a to 108 e of theprobe 108. Since the distance between the element 108 c and the lightabsorber 105 is shorter in comparison to the distance between theelement 108 a and the light absorber 105, the time that the acousticwave 106 is received by the element 108 c will be faster than the timethat the acoustic wave 106 is received by the element 108 a.Accordingly, it can be seen that the signals of the timeframe to bereceived from the voxels or pixels to be subject to imagereconfiguration are different according to the position of the elementsin the probe.

FIG. 2B is a diagram explaining the relationship of the propagatingsignals. t=0 is the base time. At t=t1, a signal 109 c is received bythe element 108 c. Reference numerals 109 a to 109 e represent thesignals of acoustic waves corresponding to the elements 108 a to 108 e.The received signal from the probe 108 is stored in the memory 110 b viathe conversion unit 110 a.

FIG. 3A is a diagram schematically showing the state where the acousticwave is reflected or transmitted at the interface of the object and theprobe. In FIG. 3A, Z₁ is the acoustic impedance of the object 104, andZ₂ is the acoustic impedance (impedance of the acoustic matching layer)of the probe 108. Moreover, θ₁ is the angle that the acoustic waveenters the probe, θ₂ is the angle of the acoustic wave after enteringthe probe, c₁ is the acoustic velocity of the object, and c₂ is theacoustic velocity of the probe. A part of the acoustic wave 106 thatpropagated in the object becomes the reflected wave 107, and theremainder enters the probe.

FIG. 3B is a graph showing the relationship of the angle θ₁ that theacoustic wave enters the probe and the reflectance R. Here, the valuesrelated to the object and the probe were calculated as follows; namely,Z₁=1.5×10⁶ kg/m²s, c₁=1500 m/s, Z₂=1.8×10⁶ kg/m²s, and c₂=2100 m/s. Asevident from FIG. 3B, the reflectance R is dependent on the angle θ₁that the acoustic wave enters the probe. Generally speaking, since thereflectance R will increase when the incidence angle θ₁ of the acousticwave is large, the receiving intensity of the acoustic wave willdecrease as the incidence angle of the acoustic wave entering theelement is large.

The flow of the reflectance loss correction processing as the uniquefeature of this invention to be performed by the correction operationunit 110 d is now explained with reference to FIG. 4. As the premises,let it be assumed that light from the light source is irradiated on aregion to be subject to image reconfiguration within the object, and theacoustic wave generated thereby is being received by the respectiveelements of the probe. The image reconfiguration by the processor isstarted from the foregoing state.

(Step S101) The correction operation unit 110 d selects arbitrary pixelsor voxels from the region to be subject to image reconfiguration. Here,an example where the voxels are selected and a three-dimensionalreconfiguration is to be performed is explained.

(Step S102) The correction operation unit 110 d selects signals that canbe used for reconfiguring the selected voxels. This process correspondsto selecting the element of the probe that can receive the acoustic wavefor use in the reconfiguration based on the position of the voxels.

(Step S103) The correction operation unit 110 d calculates the timerange based on the acoustic wave derived from the voxels selected instep S101 among the selected signals based on the arrival time of theacoustic wave caused by the positional relationship shown in FIG. 2B.

(Step S104) The correction operation unit 110 d acquires the data inputin the first table 110 c. Here, let it be assumed that the acousticimpedance and acoustic velocity of the object and the probe are input tothe first table 110 c.

(Step S105) The correction operation unit 110 d obtains the angle fromthe positional relationship of the voxels and the elements, andthereafter calculates the reflectance R of the acoustic wave on theprobe surface. The reflectance R is calculated based on Formulas (1) and(2) based on the data input to the first table 110 c and the angle θ.R=|(Z ₁ cosθ₂ −Z ₂ cos θ₁)/(Z ₁ cos θ₂ +Z ₂ cos θ₁)|²   (1)θ₂=sin⁻¹(c ₂ sin θ₁ /c ₁)   (2)

Note that, while this embodiment used the acoustic impedance and theacoustic velocity of the object and the probe, since the acousticimpedance is the product of density and acoustic velocity, it willsuffice so as long as at least two types of data among acousticimpedance, density, and acoustic velocity of the object are input to thefirst table 110 c.

(Step S106) The correction operation unit 110 d corrects the reflectanceloss of the selected signals according to the incidence angle θ₁ of theacoustic wave. The correction is performed by dividing the intensity ofthe signals of the corresponding time range by 1−R. However, the divisoris not limited to 1−R and, for instance, may also be 1−aR (wherein a isthe coefficient of a≅1, and can be adjusted by the operator). When theacoustic wave is entirely reflected by the probe surface; that is, uponsatisfying Formula (3) below, then R=1, and the signal in the foregoingcase is not used in the image reconfiguration.θ₁≧sin⁻¹(c ₂ /c ₁)   (3)

Moreover, when R is close to 1, then 1−R becomes close to 0, and noisecomponents will increase in addition to the signals. In the foregoingcase, the upper limit of 1/(1−R) is set in advance, and the upper limitis used upon exceeding the upper limit. Moreover, measures may be takenso that a signal that exceeded the upper limit is not used in the imagereconfiguration.

(Step S107) If the correction of all signals to be used in reconfiguringthe selected voxels is not complete, the routine returns to S102 and thesubsequent signal is corrected. If the correction is complete, theroutine proceeds to the subsequent processing.

(Step S108) The correction operation unit 110 d uses the correctedsignal and reconfigures the image based on well-known methods such asFBP (Filtered Backprojection) or DELAY-AND-SUM.

Note that the timing of acquiring data from the first table, timing ofselecting the signals to be used in the reconfiguration and calculatingthe reflectance at the interface, timing of performing reconfigurationand the like are not limited to the foregoing explanation. For example,it is also possible to acquire data from the first table immediatelyafter the start of processing and manage such data in the memory, anduse the data in subsequent calculations.

(Step S109) If the reconfiguration of all voxels in the region to besubject to image reconfiguration is not complete, the routine returns toS101 and the subsequent voxel is selected. Meanwhile, if thereconfiguration of all voxels in the intended region is complete, theprocessing is ended. The generated data is stored as object information.In addition, image correction processing or enhancement for achieving afavorable image display may also be performed.

Note that while this embodiment used Mathematical Formulas (1) and (2),the mathematical formula is not limited thereto, and an approximateequation may also be used. For example, as an approximate equationrelative to Mathematical Formula (1) , a formula of multiplying A (A≅1and an adjustable coefficient) to the right side of Mathematical Formula(1) may also be used.

Moreover, while this embodiment illustrates an example of disposing aplurality of probes on the object surface, since the same effect can beobtained so as long as acoustic waves can be received, it is alsopossible to use a probe configured from one element and scan the objectsurface one-dimensionally or two-dimensionally.

According to the configuration of this embodiment described above, it ispossible to reduce the difference in the receiving sensitivity in thefront direction of the probe and the receiving sensitivity in theperipheral direction of the probe by correcting the reflectance loss ofacoustic waves on the probe surface, and substantially broaden theaperture of the probe. Consequently, since received signals from variousangles will contribute upon the image reconfiguration, the resolutioncan be improved.

<Embodiment 2>

Embodiment 1 explained a configuration of correcting, for each voxel orpixel, the reflectance loss on the probe surface based on the acousticimpedance and the acoustic velocity of the object stored in the firsttable 110 c. In this embodiment, the method of correcting thereflectance loss by using an amplification factor for each receivingposition corresponding all voxels or pixels in the image reconfigurationregion is explained.

FIG. 5A is a diagram schematically showing the configuration of thephotoacoustic signal acquiring apparatus of this embodiment, and, sincethe configuration other than the processor 110 is the same as FIG. 1,the explanation thereof is omitted. A second table 110 e of FIG. 5A isinput with the amplification factor for each probe which is required forperforming image reconfiguration to all voxels or pixels in the imagereconfiguration region. In other words, the table 110 e stores, for eachangle that can be formed based on the device configuration, theamplification factor according to the positional relationship (distanceand angle) of the element and the voxels or pixels to be processes. Thisamplification factor can be calculated by using Formula (1) ofEmbodiment 1 if the acoustic velocity and impedance of the object andthe probe and the incidence angle of the acoustic wave are set inadvance.

The correction flow of this embodiment is now explained with referenceto FIG. 5B. This flow differs only with respect to the point that stepS201 is executed in substitute for steps S104 and S105 in FIG. 4 ofEmbodiment 1. Accordingly, the explanation of portions other than thosewhich are different from Embodiment 1 will be brief.

(Steps S101 to S103) The correction operation unit 110 d selectsarbitrary voxels and calculates the time range of signals to be used forreconfiguring the selected voxels.

(Step S201) The correction operation unit 110 d acquires correction datafrom the second table 110 e. As described above, the correction datastores appropriate amplification factors based on the positionalrelationship of the selected voxels and the selected elements, and thereflectance obtained from characteristics such as the acoustic velocityand acoustic impedance of the object and the probe. Thus, as a result ofperforming amplification processing to the selected signals by using theforegoing amplification factor, the reflectance loss can be corrected.Note that it is also possible to simply store the reflectance insubstitute for the amplification factor.

(Steps S106 to S109) The correction operation unit 110 d reconfiguresthe voxels using the signals that were subject to amplificationprocessing, and ends the processing upon completion of thereconfiguration of all voxels contained in the target region.

In the configuration of this embodiment described above also, theresolution is improved since the reflectance loss on the probe surfacecan be corrected. Since the amplification factor required for correctingthe reflectance loss is known in advance for each voxel or pixel and canbe easily acquired by referring to a table, the processing time can beshortened even more since there is no need to calculate formulas eachtime as with Embodiment 1.

<Embodiment 3>

This embodiment explained a configuration of correcting thedeterioration in sensitivity caused by the aperture of the probe. Thecorrection method is the same as the method of correcting thereflectance loss on the probe surface. In other words, information ofthe probe is stored in advance in the first table 110 c. In addition,the correction operation unit 110 d performs amplification processing tothe signals of the time range to be received from the arbitrary voxelsor pixels according to the incidence angle θ₁ of the acoustic wave.

Specifically, when the elements of the probe are rectangular, thedirectionality A(θ) can be obtained with Formula (4) below. Here, krepresents the wave number, a represents the element size of the probe,and θ represents the angle that the acoustic wave enters the probe.Based on this formula, the coefficient relative to the signal thatenters perpendicularly in cases where the acoustic wave enters theelement at an angle θ can be obtained.A(θ)=|{sin(ka sin θ)}/(ka sin θ)|  (4)

The correction flow of this embodiment is now explained with referenceto FIG. 6. Note that the explanation of portions other than those whichare different from FIG. 4 of Embodiment 1 will be brief.

(Steps S101 to S103) The correction operation unit 110 d selects thearbitrary voxels and calculates the time range of signals to be used forreconfiguring the selected voxels.

(Step S104) In this embodiment, the correction operation unit 110 dacquires from the first table 110 c, in addition the data (for instance,acoustic impedance and acoustic velocity) for obtaining the reflectance,data (for instance, wave number and element size) to be used incalculating the directionality.

(Step S105) The correction operation unit 110 d obtains the angle fromthe positional relationship of the voxels and elements, and thereaftercalculates the reflectance R of the acoustic wave on the probe surfacebased on Formulas (1) and (2).

(Step S301) In this embodiment, the correction operation unit 110 dcalculates the directionality A(θ) of the probe based on Formula (4).

(Step S106) In this embodiment, in addition the amplification processingbased on the reflectance R, the correction operation unit 110 d alsoperforms the amplification processing based on the directionality A(θ)to correct the signals. The signals of the time range to be obtained areforemost corrected by dividing the intensity by 1−R, and additionallydividing the product by A(θ).

However, the value used for correcting the aperture is not limited toA(θ), and, for instance, bA(θ) (wherein b is a coefficient of b≅1, canbe arbitrarily adjusted by the operator) may also be used. Moreover,when the element size is large, an upper limit may be set todirectionality 1/A(θ) in order to avoid the influence of noise frombecoming too great.

(Step S107 to S109) The correction operation unit 110 d reconfigures thevoxels using the signals that were subject to amplification processing,and ends the processing upon completion of the reconfiguration of allvoxels contained in the target region. Information of the object in theregion is thereby imaged.

According to the configuration of this embodiment described above, amore accurate image can be obtained since the directionality of theprobe is corrected in addition to the reflectance loss of the acousticwave on the probe surface being corrected.

Note that, rather than performing calculations each time using Formula(4), it is also possible to measure the directionality of the probe inadvance and store the result in the first table 110 c, and use suchmeasured directionality as needed.

<Embodiment 4>

When it is desirable to fix and measure an object or perform scanningwith the probe on the object, the object may be held with a holdingplate. In the foregoing case, the acoustic wave will be received by theprobe via the holding plate that is compressing the object.Consequently, since reflectance loss of the acoustic wave will arise atthe interface of the object and the holding plate and at the interfaceof the holding plate and the probe, there is an additional problem inthat an accurate image cannot be obtained.

Thus, this embodiment explains a configuration example where, uponholding the object with the holding plate, the reflectance loss of theacoustic wave at the interface of the object and the holding plate andat the interface of the holding plate and the probe is corrected inorder to obtain an accurate image.

FIG. 7 shows a configuration of the photoacoustic signal acquiringapparatus according to Embodiment 3 of this invention. The photoacousticsignal acquiring apparatus comprises a light source 201, an irradiationoptical system 202 such as a mirror, a probe 208, and a processor 210.The same components as Embodiment 1 may be used for the foregoingcomponents.

The photoacoustic signal acquiring apparatus of this embodimentcomprises a holding plate 211. The holding plate 211 may be fixed ormovable. Otherwise, when there are two holding plates, one may be fixedwhile the other one made to be movable so as to compress and hold theobject at an appropriate thinness. The holding plate on the light sourceside is preferably made of polycarbonate or acrylic which transmitslight easily, and the holding plate on the probe side is preferably madeof polymethylpentene which easily transmits acoustic waves.

In this embodiment, the reflectance loss of the acoustic wave at theinterface of the object and the holding plate and at the interface ofthe holding plate and the probe is corrected. Thus, the first table 210c stores at least two among acoustic impedance, density, and acousticvelocity data related to the object, holding plate and probe.

As the correction method, after obtaining the reflectance with thecalculation method described in Embodiment 1, the processing ofamplifying the signal intensity by dividing the signal intensity by thereflectance may be applied to the respective interfaces.

Moreover, as with Embodiment 2, the processor 210 may also include asecond table 210 e (not shown) containing the amplification factor ofeach element of the probe corresponding to all voxels or pixels in theimage reconfiguration region.

According to the configuration of this embodiment explained above, evenin cases where the object is held by the holding plate, the reflectanceloss of the acoustic wave at the interface of the object and the holdingplate and at the interface of the holding plate and the probe iscorrected, and it is thereby possible to obtain an image with improvedresolution.

<Embodiment 5>

While the reflectance loss of the acoustic wave at the interface of theobject and the holding plate and at the interface of the holding plateand the probe was corrected in Embodiment 4, if the attenuation of theacoustic wave in the holding plate is also obtained and the attenuationis corrected, information in the object can be more accurately imaged.This embodiment explained this method with reference to FIG. 8.

In FIG. 8, reference numeral 204 represents an object, reference numeral211 represents a holding plate, reference numeral 208 represents aprobe, and the acoustic wave 206 enters the probe from the object viathe holding plate. When the thickness of the holding plate is d, theabsorption coefficient is μ, and the angle that the acoustic wave entersthe probe from the holding plate is θ, the relationship of the intensityS₀ of the acoustic wave immediately after entering the holding plate andthe intensity S₁ of the acoustic wave immediately before being emittedfrom the holding plate will be as shown in Formula (5).S ₁ =S ₀exp(−μd/cos θ)   (5)

Accordingly, in Embodiment 4, upon performing the correction of dividingthe signal intensity by the reflectance, if the signal intensity issimultaneously divided by exp(−μd/cos θ), it is possible to correct theattenuation of the acoustic wave in the holding plate according to theattenuation rate, and thereby acquire a more accurate image.

According to the configuration of this embodiment explained above, sincethe attenuation of the acoustic wave in the holding plate is corrected,it is possible to obtain a more accurate image. Note that while thisembodiment used Mathematical Formula (5), the mathematical formula isnot limited thereto, and an approximate equation may also be used.

<Embodiment 6>

While Embodiments 1 to 5 used an estimate value as the acousticimpedance of the object, the acoustic impedance may differ for eachobject. Accordingly, in order to obtain a more accurate image, it isdesirable to actually measure the acoustic impedance for each object andcorrect the reflectance loss of the acoustic wave by using the resultthereof. This embodiment explains a configuration example of actuallymeasuring the acoustic impedance of the object and correcting thereflectance loss based on the result of such actual measurement.

FIG. 9A is a diagram showing a configuration of the photoacoustic signalacquiring apparatus according to this embodiment. The photoacousticsignal acquiring apparatus comprises a light source 301, an irradiationoptical system 302 such as a mirror, a probe 308, a processor 310, and aholding plate 311. The same components used in the foregoing embodimentsmay also be used as the foregoing components. Moreover, thephotoacoustic signal acquiring apparatus in this embodiment comprises atransmitting circuit 312 for transmitting ultrasound waves from theobject 304 to the probe 308. As a result of a transmitting signal 313being transmitted from the transmitting circuit 312 to the probe 308,ultrasound waves are transmitted from the probe 308.

The method of measuring the acoustic impedance of the object is nowexplained with reference to FIG. 9B. Foremost, a pulse transmission wave314 of a certain intensity is caused to enter from the probe toward theholding plate and the air interface in a state where the holding plateis not holding the object and is in contact with air. This ultrasoundwave intensity shall be S₂. Since the ultrasound wave is substantiallyreflected by the holding plate and air interface in its entirety, anultrasound wave (first reflected wave) of an intensity that issubstantially the same as the incoming ultrasound wave intensity S₂ isreceived by the probe.

Subsequently, the ultrasonic wave is transmitted in a state where theholding plate is holding the object. At base time t=0, the same pulsetransmission wave 314 of intensity S₂ is caused to enter the object 304from the probe 308 via the holding plate 311. Consequently, the incomingpulse transmission wave 314 is reflected at the interface of the objectand the holding plate (second reflected wave).

Here, if the acoustic velocity c in the holding plate is known, it ispossible to calculate the time (t=2 d/c) required for the ultrasoundwave to reciprocate the holding plate 311 having the thickness d. Thus,the intensity S₃ of the pulse reflected wave 315 that reaches the probewhen time t elapses from the base time is detected. The reflectanceR=S₃/S₂ can thereby be calculated.

When the acoustic impedance of the object is Z₁, and the acousticimpedance of the holding plate is Z₂, the relationship of thereflectance R and Z₁, Z₂ will be as shown in Formula (6).R=|(Z ₂ −Z ₁)/(Z ₂ +Z ₁)|²   (6)

Accordingly, if the acoustic impedance Z₂ of the holding plate is known,it is possible to measure the reflectance R, and thereafter calculatethe acoustic impedance Z₁ of the object 304. The calculated acousticimpedance of the object 304 is stored in the table 310 c.

Subsequently, the image reconfiguration according to the methoddescribed in Embodiment 1 is performed based on the stored acousticimpedance of the object 304.

According to the configuration of this embodiment explained above, sincethe acoustic impedance of the object can be actually measured, thereflectance loss is corrected more accurately, and it is possible toobtain an image with improved resolution.

Note that, without limitation to the first table 310 c during thecorrection, the second table containing the correction data of therespective elements of the probe corresponding to all voxels or pixelsin the image reconfiguration region may also be used. In the foregoingcase, after obtaining the acoustic impedance of the object 304 with themethod of this embodiment, data to be used for the correction iscalculated based on the positional relationship of the voxels or pixelsand the elements of the probe. In addition, the correction data isstored in the second table 310 e, and used upon performing the imagereconfiguration.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-196049, filed on Sep. 8, 2011, which is hereby incorporated byreference herein its entirety.

What is claimed is:
 1. An object information obtaining apparatus,comprising: a probe configured to receive an acoustic wave which isgenerated within an object irradiated with light and to output atime-series signal; and a processor configured to obtain objectinformation at a voxel or a pixel, based on the signal, wherein saidprocessor corrects an intensity of the time-series signal derived fromthe voxel or the pixel, by performing correction corresponding to areflectance loss of the acoustic wave which is obtained according to anangle of the acoustic wave entering said probe, and wherein saidprocessor obtains the object information at the voxel or the pixel basedon the corrected intensity.
 2. The object information obtainingapparatus according to claim 1, wherein, when a reflectance upon theacoustic wave entering said probe is R, and a coefficient is a, saidprocessor performs the correction by dividing the intensity derived fromthe voxel or the pixel, of the time-series signal, by (1−aR).
 3. Theobject information obtaining apparatus according to claim 2, furthercomprising: a first table storing at least two among acoustic impedance,density and acoustic velocity in relation to the object and said probe,wherein said processor obtains the reflectance by performing calculationbased on information stored in said first table.
 4. The objectinformation obtaining apparatus according to claim 1, furthercomprising: a second table storing a correction factor to be used forcorrecting the intensity of the received signal for each angle of theacoustic wave entering said probe, wherein said processor corrects, bythe correction factor, the intensity derived from the voxel or thepixel, of the time-series signal.
 5. The object information obtainingapparatus according to claim 1, wherein said processor corrects theintensity derived from the voxel or the pixel, of the received signal byperforming correction corresponding to a sensitivity of said proberelative to the acoustic wave which deteriorates according to the angleof the acoustic wave entering said probe and in addition performingcorrection corresponding to the reflectance loss of the acoustic wave.6. The object information obtaining apparatus according to claim 1,further comprising: a holding member configured to hold the object,wherein said probe receives the acoustic wave from the object via saidholding member, and said processor corrects the intensity derived fromthe voxel or the pixel, of the time-series signal by performingcorrection corresponding to a reflectance loss obtained according to anangle of the acoustic wave entering said holding plate member inaddition to performing correction corresponding to the reflectance lossof the acoustic wave which is obtained according to the angle of theacoustic wave entering said probe.
 7. The object information obtainingapparatus according to claim 6, wherein said processor obtains anattenuation rate of the acoustic wave in said holding member based on athickness and an absorption coefficient of said holding member, andperforms correction of the intensity derived from the voxel or pixel, ofthe time-series signal based on the attenuation rate.
 8. The objectinformation obtaining apparatus according to claim 6, wherein said probetransmits an acoustic wave to the holding member in a state in whichsaid holding member is not holding the object and receives a firstreflected wave, and thereafter transmits an acoustic wave to saidholding member in a state in which said holding member is holding theobject and receives a second reflected wave, and said processor obtainsthe reflectance loss by using intensities of the first and secondreflected waves and an acoustic impedance of the object obtained from anacoustic impedance of said holding member.
 9. An object informationobtaining method, for obtaining object information at a voxel or a pixelbased on an intensity derived from the voxel or the pixel, of atime-series signal obtained by receiving by means of a probe an acousticwave which is generated within an object irradiated with light,comprising: correcting the intensity of the time-series signal derivedfrom the voxel or the pixel, by performing correction corresponding to areflectance loss of the acoustic wave which is obtained according to anangle of the acoustic wave entering the probe, and obtaining the objectinformation at the voxel or the pixel based on the corrected intensityobtained in said correcting step.
 10. An apparatus, comprising: a probeconfigured to receive an acoustic wave which is generated within anobject irradiated with light and output a time-series signal; and aprocessor configured to obtain object information at a voxel or a pixelbased on the time-series signal, wherein said processor corrects anintensity of the time-series signal, derived from the voxel or the pixelbased on an angle of the acoustic wave entering said probe, at least twoamong acoustic impedance, density, and acoustic velocity in relation tothe object, and at least two among acoustic impedance, density, andacoustic velocity in relation to said probe, and wherein said processorobtains the object information at the voxel or the pixel based on thecorrected intensity.
 11. An object information obtaining method forobtaining object information at a voxel or a pixel based on atime-series signal obtained by receiving, with a probe, an acoustic wavewhich is generated within an object irradiated with light, comprising:correcting an intensity, of the time-series signal, derived from thevoxel or the pixel based on an angle of the acoustic wave entering theprobe, at least two among acoustic impedance, density, and acousticvelocity in relation to the object, and at least two among acousticimpedance, density, and acoustic velocity in relation to the probe, andobtaining the object information at the voxel or the pixel based on thecorrected intensity.